Metallic feedthroughs (e.g., electrodes) are well known in the industry and are commonly used to provide electrically conductive paths through ceramic articles while excluding the passage of solids, liquids, and gases. One application for such feedthroughs is in a magnetic inductive field flow meter which requires a chemically inert electrode hermetically sealed in the wall of an electrical insulating and chemically resistant ceramic cylinder. Feedthroughs are also used to carry electrical power to/from an active device such as a transistor or diode.
Various commercial methods have been employed to produce metallic feedthroughs. Typically, with alumina ceramics, the feedthroughs (i.e., metallic conductors) are brazed in place using the following multi-step process: As illustrated in FIG. 1, a ceramic article 1 has an ceramic electrode insertion hole 2 which is coated with a moly-manganese or tungsten paint 3 (i.e., ceramic metallization) and sintered at approx. 1,500.degree. C. in wet hydrogen. The inside diameter of the coated insertion hole 2 is then nickel plated 4 and sintered at approx. 950.degree. C. (i.e., metallization plating). The metallic conductor 5 is inserted in the electrode insertion hole 2 with a brazing alloy preform 6 or a paste positioned on the top side to fill voids at the interface of the conductor 5 and the insertion hole 2 upon brazing. The ceramic article 1 and conductor 5 are then simultaneously heated to effectuate brazing of the conductor 5. A major drawback of this method is that, in most instances, the conductor 5 is placed in a vertical position during brazing to avoid migration of the brazing alloy or paste (disposed on the top side of the conductor 5) from regions of the interface under the influence of gravity. This multi-step operation is also very time consuming and expensive.
In U.S. Pat. No. 5,095,759 a method is disclosed wherein a core wire (e.g., platinum) 9, coated with an active alloy paste 7, is inserted into a pre-sintered ceramic insertion hole 8. (See FIG. 2.) The ceramic article 10 and the wire 9 are then simultaneously heated to braze the wire 9 in the hole 8. According to this method, since the paste 7 (i.e., powder) only occupies approx. 50% of the space, an additional alloy, in the form of a brazing filler preform 11 is placed on the top side of the ceramic article 10 to fill the void upon brazing. This method also suffers from similar drawbacks, as noted above.
In U.S. Pat. No. 4,912,838 a method is disclosed wherein an electrode is formed by filling a predetermined hole 12 in a sintered ceramic 13 with a conductive metal paste 14 comprising moly-manganese, tungsten, copper, or similar material and subsequently heating the paste 14 to promote adherence to the ceramic (see FIG. 3). This method often produces conductive feedthroughs that fail during vacuum leak testing.
In Japanese Patent (Laid-open) No. 58-501552 (equivalent to U.S. Pat. No. 4,507,975), another method is disclosed wherein a high melting point but ductile metal pin is inserted into an electrode insertion hole of a cylindrical molded body made of a non-sintered ceramic material (e.g., oxide ceramic). In this state, the molded body is sintered at a predetermined temperature and at the same time the electrode is integrally fixed to the electrode insertion hole by sintering. That is, since a ceramic material contracts (approx. 17 to 20% in the case of Al.sub.2 O.sub.3) by sintering, the electrode and the molded body are integrally formed, and a liquid-tight seal of the electrode insertion hole is obtained. A noble metal such as platinum or a platinum alloy is used as the electrode material.
According to the noted method, however, in order to reliably seal the electrode insertion hole, the dimensional tolerance between the electrode and the insertion hole, and the mating surface finishes must be strictly controlled. Further, since the non-sintered electrode insertion hole is weak, it is difficult to machine without damage, resulting in a large number of manufacturing steps, and subject to additional damage when the electrode is inserted into the non-sintered electrode insertion hole. Finally, variations in the contraction rate of the ceramic material upon sintering, due to differing lots or manufacturing time, generally result in a defective liquid seal or undesirable residual stresses which could damage the ceramic article upon sintering.
It is therefore an object of the present invention to provide a one step method to produce a sealed conductive feedthrough in a ceramic material.